A liquid crystal display device is basically provided with a circuit array substrate, a counter substrate and a liquid crystal layer held between the circuit array substrate and the counter substrate. The circuit array substrate contains switching elements to operate pixels. Although amorphous-silicon thin-film transistors are still applicable to prior art switching elements, poly-silicon thin-film transistors have been recently developed to replace the amorphous-silicon thin-film transistors and put into practical use.
In order to make the poly-silicon thin-film transistors on the circuit array substrate, an amorphous-silicon film is formed on an insulation substrate, such as a glass substrate, the amorphous-silicon film is then annealed to re-crystallize as a poly-silicon film, and a patterning process is subsequently carried out to make the poly-silicon film an active layer for the poly-silicon thin-film transistors.
Next, a low-density P-type or N-type impurity is doped into the active layer (the “lightly doped active layer”) to control a threshold voltage of the poly-silicon thin-film transistors and a gate insulation layer is formed on the insulation substrate containing the lightly doped active layer.
Further, an electrically conductive film is coated on the gate insulation layer and a patterning process is performed for the conductive film and gate insulation layer to form gate electrodes on the active layer. A high-density P-type or N-type impurity is doped into the active layer to form source and drain regions of the poly-silicon thin-film transistors.
Subsequently, after an interlayer insulation film is coated on the gate insulation layer and the gate electrodes, contact holes are made through the interlayer insulation film so as to reach the source and drain regions, respectively. The interlayer film and contact holes are covered with an electrically conductive film to which a patterning process is applied to form source and drain electrodes and signal lines.
Next, a protective film is coated on the source and drain electrodes and the interlayer film and contact holes are made through the protective film so as to reach the drain electrodes, respectively. The contact holes are filled with an electrically conductive film to connect the drain electrode to an external device. In this way, the circuit substrate is completed for the liquid crystal display device.
Additionally, data entry devices, such as photo-electric sensor elements, are provided on the glass substrate. The photo-electric sensor element may consist of a PIN photo-electric diode in which an I (intrinsic) layer generates photons on receipt of incident light.
As described above, the impurity is necessarily doped into the active layer for the thin-film transistor to obtain prescribed performance characteristics. In order to increase photo-electric sensitivities of the PIN diode, however, it is desirable that no impurity is doped into the I layer of the PIN diode.
Where the thin-film transistors are formed on the glass substrate in the same processes chamber as the photo-electric sensor elements, the impurity concentration cannot be changed. Thus, it is not easy to form thin-film transistors with prescribed performance characteristics and sensitivity-enhanced photo-electric sensor elements simultaneously. Those thin-film transistors and photo-electric sensor elements can be formed in separate processes in which different density impurities are doped into the active layers of the thin-film transistors and the I layers of photo-electric sensor elements, respectively. Such a manufacturing method, however, results in increase in the number of manufacturing processes and cannot be made easy and inexpensive in manufacturing cost.
Since, on the other hand, the poly-silicon active layer has high turned-on resistance, not only such pixel switching circuits but also driver circuits can be made of the poly-silicon active layer formed on the array substrate. In that case, however, the channel length of a poly-silicon thin-film transistor in the driver circuit should be short because an operating frequency of the driver circuit is usually required to be higher as the liquid crystal display device becomes finer in image resolution.
Generally, a withstand voltage at the drain electrode becomes lower as the channel length is set to be shorter. In other words, when the channel length is shorter, carriers are more concentrated at the drain electrode where the electric field is intensive, and drain avalanches are caused at a lower voltage. In fact, a very-large-scale-integration (VLSI) circuit has similar situations to it. That is, its channel length is shortened for high frequency operations but the VLSI circuit is used at a low power source voltage in compliance with its lowered withstand voltage. Such prior art technology is disclosed in Japanese Patent No. 2959682 (see its descriptions on pages 2 through 4 and FIG. 1).
A liquid crystal display device or an electro-luminescence display device is required to drive liquid crystal elements or electro-luminescence elements at a prescribed voltage. Thus, its entire power source voltage cannot be lowered, e.g., a liquid crystal element must be driven at a voltage not less than 10V.
In order to satisfy such contrary requirements that a display device operates at a high frequency with a high withstand voltage, thin-film transistors with different channel lengths are formed on a glass substrate. Those with short and long channel lengths are used for a high frequency operation and for a high withstand application, respectively.
FIGS. 27 and 28 show characteristics of drain current Id-gate voltage Vg (the “Id-Vg characteristics”) in accordance with channel lengths where a poly-silicon thin-film is used for an active layer. According to the Id-Vg characteristics, as the channel length becomes shorter, the threshold voltage reduces and a turned-off current increases at the gate voltage Vg=0V. In short, these result from the fact that the active layer is an amorphous-silicon thin-film and maintains a middle band-gap level. Here, the turn-off current increase brings about increase in currents consumed in circuits or results in error circuit operations. Therefore, the mere change of channel lengths does not necessarily make thin-film transistors operate at high frequency or at a high withstand voltage.